How to add round rim around object?
Start with a loop cut around your object:

Give the loop some thickness by beveling it with Ctrl+B. Select the new faces around the edge (but not on the existing extrusion), and &quot;extrude faces along normals&quot;.

Select the top and bottom edges of the new extrusion:

Then use Ctrl+B to bevel again, this time scrolling the mouse wheel to add more segments. Move the mouse to adjust the bevel until it looks right.

If you're concerned about clean topology, you may need to add some new cuts into the extrusion that was there before the loop, and remove the new overlapping face created by the extrusion, but otherwise it should be fine.